Untreated optical-quality windows for optical and infrared sensors and for laser transmit/receive systems are often electrically clear, having no exterior surface conductivity and therefore unable to shield sensitive internal components from electromagnetic interference (EMI) or electromagnetic pulses (EMP). A conductive surface coating applied to a sensor window may be used to pass optical energy while blocking EMI/EMP energy, by conducting it directly to the surrounding window frame. Such a conductive coating is typically made of metal or metallic film. Continuous metal films have reasonably good visible-light transmission, but have poor to zero infrared transmission. Therefore, the metallized windows may meet EMI/EMP requirements, but cannot meet broadband optical/infrared transmission requirements.
Conventionally known solutions widely used in the aircraft and electromagnetic industry involve the application of straight-line wire meshes or photolithographic grids to the optical window glass. The open spaces between the wires or grid lines do not affect the passage of visible light and infra-red energy through the sensor window, while the wires or grid lines, which are thermally and visually opaque, conduct the majority of EMI/EMP RF energy away to the window frame. These straight-line meshes and grids provide adequate EMI/EMP shielding, but the grid and wire patterns they employ generate significant scattering and diffraction of transmitted optical and IR energy. These traditional approaches also had problems when operated away from normal incidence angles, as light transmission dropped and optical scattering increased rapidly with increasing incidence angle.
In searching for improvements to the above-stated problems, Lockheed Martin developed a new form of conductive pattern termed “hub-spoke” in the early 1990's that was a hybrid of very small metallic film circles, deposited 1 to 2 microns thick, with 10 micron widths and 200-300 micron circle diameters, with straight-line interconnectors. This Lockheed Martin-originated pattern was implemented at Battelle Laboratories, Columbus Ohio, and has been widely applied to a number of military platforms. The center positions for each of the circles (termed “hubs”) was slightly randomized, and the circles did not overlap, but were instead electrically connected with very short straight-line segments, termed “spokes”, having angles that were randomized as well. This new grid pattern greatly reduced optical scattering and diffraction relative to earlier straight-line meshes and grids, while providing the equivalent EMI/EMP shielding.
The hub-spoke arrangement, however, was still subject to particular diffraction effects associated with grid-like structures. Specifically, the hub-spoke pattern contained many straight-line segments and had a relatively regular pattern. Off-normal operation, even with this pattern, continued to produce significant drops in light transmission and increases in scattering. Despite having a less regular and less angular arrangement, the hub-spoke pattern still contained many of the disadvantages of predecessor grids, albeit to a lesser degree.